The present invention relates to a fixing device used for image forming devices based on electrostatic-recording or electro-photographic recording such as copying machines, facsimiles and printers. The present invention further relates to a toner image fixing device using the electromagnetic induction heating method.
Demands for faster and more energy-efficient image forming devices such as printers, copying machines and facsimiles have been increasing in the market. To satisfy such demands, it is critical to improve the thermal efficiency of fixing devices used in the image forming devices.
During image forming processes such as electro-photographic recording, electrostatic recording and magnetic recording, an image forming device forms an unfixed toner image on recording media such as recording sheets, sensitized paper and electrostatic recording paper by an image transfer method or a direct method. The unfixed toner image is fixed, in general, by a fixing device based on contact heating methods such as a hot roller method, a film heating method, or an electromagnetic induction heating method.
The fixing device of the hot roller method comprises, as a basic construction, a pair of rollers including a temperature regulated fixing roller having a heat source such as a halogen lamp and a press roller pressing against the fixing roller. A recording medium is inserted into and carried through a section where the fixing roller and press roller come into contact, a so-called fixing nip region, so that the unfixed toner image is melted and fixed by heat and pressure applied by the rollers.
The fixing device of the film heating method is disclosed, for example, in the Japanese Patent Laid-Open Publications S63-313182 and H01-263679.
In the case of the foregoing fixing device, a recording medium is positioned into a close contact with a heater which is tightly fixed to a supporting member via a thin heat-resistant fixing film. The fixing film is slid against the heating body and the heat is transferred from the heating body to the recording medium via the film. The heater in the fixing device is a ceramic heater constructed such that a resistor layer is disposed on a heat-resistant substrate having insulation property and high heat conductivity such as alumina (Al2O3) or aluminum nitride (AlN). Since this fixing device uses the thin fixing film with a low heat capacity, its heat conductivity is higher than that of the fixing device using heated rollers. Thus this fixing device achieves a shorter warm-up time, a quick-start and improved energy efficiency.
The Japanese Patent Laid-Open Publication H08-22206 discloses a fixing device based on the electromagnetic induction heating method. According to the method, a Joule heat produced by an eddy current generated in a magnetic metal member by an alternating field, heats up a heater including the metal members by an electromagnetic induction.
The following is a description of the construction of the fixing device based on the electromagnetic induction heating.
FIG. 5 is a schematic view showing a conventional fixing device of the electromagnetic induction heating. As FIG. 5 shows, the conventional fixing device comprises a guide 21, a heater 20, a film 17, and a press roller 22. The guide 21 is disposed in the inner surface of the film 17, and the heater 20 is disposed in the guide 21. The heater includes a coil unit 18 and a magnetic metal member 19. The heat-resistant, cylindrical film 17 surrounds the guide 21 such that the magnetic metal member 19 is in contact with its inner surface. The press roller 22 forms a fixing nip region N with the film 17 by pressing against the film 17 at the location where the magnetic metal member 19 is disposed, and turns the film 17.
The film 17 is made with either:
a) a single-layer film made of PTFE, PFA or FEP; or
b) a composite layer film wherein the external surface of a film made of polyimides, polyamide-imides, PEEK, PES or PPS is coated with PTFE, PFA or FEP. The thickness of the film is not more than 100 xcexcm and, preferably, between 20 xcexcm and 50 xcexcm.
The guide 21 is composed of a material such as PEEK and PPS with rigidity and heat-resistant properties. The heater 20 is imbedded in approximately the center of the longitudinal direction of the guide 21.
The press roller 22 comprises a core 22a and a surrounding heat-resistant rubber layer 22b composed of a material such as silicon rubber that has a high releasing ability. The press roller 22 is disposed such that it presses against the magnetic metal member 19 of the heater 20, via the film 17, at a predetermined pressure through the use of bearings or other supplemental pressuring members (not illustrated). The press roller 22 rotates counterclockwise by a driving means.
The rotation of the press roller 22 causes a friction between the press roller 22 and the film 17 and it applies a rotation power onto the film 17. The film 17 slides and turns while being fixed tightly to the magnetic metal member 19 of the heater 20.
When the heater 20 reaches a predetermined temperature, a recording medium 11 having an unfixed toner image T formed at an image forming section (not illustrated) is inserted between the film 17 and the press roller 22 at the fixing nip region N. The recording medium 11 is sandwiched between the press roller 22 and the film 17, and travels through the fixing nip region N. While the recording medium 11 is traveling through the fixing nip region N, heat from the magnetic metal member 19 is applied, via the film 17, to the recording medium 11, and its unfixed toner image T is melted and fixed. At the exit of the fixing nip region N, the recording medium 11 is separated from the surface of the film 17 and brought onto a paper tray (not illustrated).
In the fixing device based on the electromagnetic induction heating method, magnetic metal member 19, an induction heating means, can be located close to the toner image T formed on the recording medium 11 via the film 17. Therefore, compared with the fixing device using the film heating method, it enjoys higher heating efficiency.
Fixing devices for the full-color image forming devices need to be able to heat and melt over four layers of toner particles. To achieve this, a fixing device of the electromagnetic induction heating method needs to employ a resilient rubber layer of 200 xcexcm in thickness on the surface of the film so that the toner image is adequately enclosed and evenly heated and melted.
However, if the film is coated with a 200 xcexcm resilient layer such as silicon rubber, the heat response lowers due to the low heat conductivity of the resilient layer. As a consequence, the difference in temperature between the inner surface of the film which is heated by the heater and the outer surface which is in contact with the toner becomes significant.
Therefore, it becomes difficult to control the temperature of the surface of the film which acts as a heating medium for the toner and has a significant influence on the fixing condition of the toner.
The present invention aims at providing a fixing device based on the electromagnetic induction heating method, which controls the temperature of the toner heating medium in a stable manner.
The fixing device of the present invention comprises a heating roller, a fixing roller, an endless belt toner, and a press roller. The heating roller is made of magnetic metal and is heated by electromagnetic induction heating and the fixing roller is disposed parallel to the heating roller. The endless toner heating medium belt is bridged across the heating roller and the fixing roller, and the belt is heated by the heating roller and rotated by the two rollers. The press roller is pressed to the fixing roller via the toner heating medium belt, and the press roller rotates in the same direction as the toner heating medium to form a fixing nip region.
According to the present invention, since magnetic metal is used for the base material of the toner heating medium belt, the toner heating medium belt is heated more efficiently by induction heating.
According to the construction of the present invention, the toner heating medium belt is sent to the fixing nip region while very small temperature differences are maintained between the inner and outer surfaces of the toner heating medium belt. Therefore, the temperature of the toner heating medium belt can be strictly controlled, so that toner images can be fixed in a stable manner.